This invention concerns a screen used in oil or water wells or the like, to prevent sand or other debris from entering the tubing used for producing oil or water.
Well screens have been used for many years to limit the production of sand and other debris in water, oil and gas wells. Well screens are simply filters that keep sand in the well, eliminating the need to separate the sand from produced fluid at some later stage in the process.
In a typical installation, a casing extends down the borehole of a well and the fluid produced is pumped from within that casing. In many wells a perforated casing may be used for recovering the fluid. In some places, there are loose particles of formation (commonly referred to as "sand", even though not technically sand) which may be entrained in the fluid. It is quite desirable to prevent such sand from entering the production tubing. Thus, a sand filter or well screen is attached at the bottom of the tubing. Such a well screen may extend many feet through a producing formation.
Some well screens are of a rather simple design: a piece of steel pipe with small openings slotted or drilled through the pipe wall. The openings machined in the pipe wall are either smaller than the sand particles in the well, or sand of a grain size larger than the openings is packed around the well screen in the borehole to serve as a filter. This "gravel pack" around the well screen filters out smaller sand particles. Other screens are more complex, using a filter medium such as porous metal, controlled grain size sand, or a fine screen to filter out the sand.
There have recently been developed fiberglass well screens that are pre-packed with sand. An exemplary pre-packed fiberglass well screen comprises inner and outer fiber reinforced epoxy tubes. The annular space between the tubes is filled with a controlled grain size sand to act as a filter medium. The inner and outer tubes are slotted with a slot width narrower than the grain size of the sand packed into the annulus.
Conventional well screens are assembled end to end with couplings to obtain a desired total length of screen in the well. These pipe couplings have a larger diameter than the well screen. It is desirable to have a well screen with a large diameter since this increases the screen area, which not only enhances flow rate through the screen, but also can prolong the screen lifetime before the filter gets plugged. A problem with prior couplings has been that the screens fitted into a well bore must be appreciably smaller than the well bore, so that the couplings can fit into the well. Thus, for example, in a 4" (100 mm) borehole, the well screen has couplings nearly the diameter of the borehole and the screen itself has an outside diameter of only about 73 mm. Larger screens cannot be used since the couplings would not fit into the 100 mm borehole.
It is therefore desirable to provide a well screen having an outside diameter approximately the same as the outside diameter of the couplings.